Growth kinetics of GaN grown by gas-source molecular beam epitaxy

We report on the incorporation kinetics of gallium during gas-source molecular beam epitaxy (GSMBE) of GaN using elemental Ga and NH3 gas as source materials. Desorption mass spectrometry (DMS) is used to perform in situ quantitative measurements of GaN formation, Ga desorption and Ga surface accumulation during growth. The fate of formation of GaN is reported as a function of incident Ga flux (0.1-0.75 ML/s) and incident NH3 flux (1-300 x 10(-7) Torr beam-equivalent pressure) at a constant growth temperature of 725 degrees C. Three distinct growth regimes are observed: (1) GaN is formed where all of the incident Ga flux is consumed, (2) part of the incident Ga is consumed to form GaN while the excess is desorbed from the surface, and (3) GaN formation coexists with desorption and surface accumulation of Ga. It is generally observed that the Ga surface accumulation is inhibited by increasing the rate of incidence of NH3 and/or by decreasing the rate of incidence of Ga at this temperature. In addition, the order of the reaction between Ga and NH3 is determined to be unity, supporting the validity of the Ga + NH3 --> GaN + 3/2 H-2 reaction.